With Google, I'm not sure; it gives good search results and experience, so I keep using it. If there's an alternative that does it better I might switch, eventually.

With Amazon it's a bit more malicious, in that they use their vast revenue and whatnot to force their prices below what any competitor could do, thus forcing them out of business.

Then again I'm sure that Google's acqui-hired any search start-up that might've become a competitor eventually.